1. Field of the Invention
The present invention relates to a Raman amplifier for causing Raman amplification of signal light and to an optical transmission system provided with such a Raman amplifier.
2. Description of the Related Art
The Raman amplifier amplifies signal light by utilizing the induced Raman scattering effect in an optical fiber to which pump light has been supplied. The Raman amplifiers have a wider gain band than rare-earth-element-doped optical fiber amplifiers. However, there continues to be a demand for the increase in the gain band of the Raman amplifiers.
To increase the gain band of the Raman amplifiers, a technique using multiplexed light having different wavelengths as pump light for Raman amplifiers is known (Y. Emori, et al., “100-nm bandwidth flat-gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM laser diode unit”, Electron. Lett., Vol. 35, No. 16 (1999)). Another technique using amplified spontaneous emission (ASE) light, which is generated by supplying pump light to an erbium (Er)-doped optical fiber, as pump light for Raman amplifiers is also known (T. Tsuzaki, et al., “Broadband Discrete Fiber Raman Amplifier with High Differential Gain Operating Over 1.65 μm-band”, OFC2001, MA3).
However, in the first technique, many light sources are required, which increases the size of the device. In the second technique, the bandwidth of the pump light for Raman amplification is only about 35 nm, which is insufficient for wider-band Raman amplifiers.